Among the stages of the HIV life cycle, viral entry is an attractive target for the design of new therapeutic agents. The principal purpose of this project is to design and characterize an efficient entry inhibitor. Additionally we have attempted to define the basic biochemical requirements necessary for efficient inhibition of HIV-1 entry into primary CD4+ T-cells. Such information should be generally useful in the design of entry inhibitors and vaccines. Monomeric soluble CD4 (sCD4) has been evaluated as a decoy drug to absorb sufficient amounts of HIV in vivo to limit the progression of disease. Unfortunately, sCD4 was ineffective in human trials. We hypothesized that a sCD4-based molecule that was large, that bound multiple gp120s simultaneously, and that was highly avid to gp120 could overcome the defects of monomeric sCD4 and effectively inhibit HIV entry into target cells. Therefore, we constructed a polymeric CD4-IgG1 fusion protein with a very large hydrodynamic radius of 12 nm and the capacity to bind at least 10 gp120 subunits with binding kinetics that suggested a highly avid interaction to virion-associated envelope; the protein was called D1D2-Igatp. Unlike sCD4, this protein did not enhance viral replication at suboptimal concentrations. In viral neutralization assays, D1D2-Igatp inhibited HIV replication at levels comparable to the most potent neutralizing antibodies available. In addition, it has been determined that D1D2-Igatp crosslinks CD16 receptors on NK cells and activates those cells. The dual specificity of this molecule for both HIV envelope and CD16 may promote NK cell-mediated killing of HIV-infected cells. These observations may aid in the design of new therapeutic strategies for HIV infection.Based on these observations we are evaluating D1D2-Ig-alpha-tp as a potential therapeutic agent and a potential adjuvant vaccine strategy.